Introduction and uniqueness
The algae are the simplest members of the plant kingdom, and the blue-green algae are the simplest of the algae. They have a considerable and increasing economic importance; they have both beneficial and harmful effects on human life. Blue-greens are not true algae. They have no nucleus, the structure that encloses the DNA, and no chloroplast, the structure that encloses the photosynthetic membranes, the structures that are evident in photosynthetic true algae. Infact blue-greens are more akin to bacteria which have similar biochemical and structural characteristics. The process of nitrogen fixation and the occurrence of gas vesicles are especially important to the success of nuisance species of blue-greens. The blue-greens are widely distributed over land and water, often in environments where no other vegetation can exist. Their fossils have been identified as over three billion years old. They were probably the chief primary producers of organic matter and the first organisms to release elemental oxygen, O2, into the primitive atmosphere, which was until then free from O2. Thus blue-greens were most probably responsible for a major evolutionary transformation leading to the development of aerobic metabolism and to the subsequent rise of higher plant and animal forms. They are referred to in literature by various names, chief among which are Cyanophyta, Myxophyta, Cyanochloronta, Cyanobacteria, blue-green algae, blue-green bacteria.
The majority of blue-greens are aerobic photoautotrophs: their life processes require only oxygen, light and inorganic substances. A species of Oscillatoria that is found in mud at the bottom of the Thames, are able to live anaerobically. They can live in extremes of temperatures -60°C to 85°C, and a few species are halophilic or salt tolerant (as high as 27%, for comparison, conc. of salt in seawater is 3%). Blue-greens can grow in full sunlight and in almost complete darkness. hey are often the first plants to colonize bare areas of rock and soil, as an example subsequent to cataclysmic volcanic explosion (at Krakatoa, Indonesia in 1883). Unlike more advanced organisms, these need no substances that have been preformed by other organisms.
At the onset of nitrogen limitation during bloom conditions, certain cells in Anabaena and Aphanizomenon evolve into heterocysts, which convert nitrogen gas into ammonium, which is then distributed to the neighboring cells of a filament. In addition, blue-greens that form symbiotic (mutually beneficial) relationships with a wide range of other life forms, can convert nitrogen gas into ammonium.
Finally, at the onset of adverse environmental conditions, some blue-greens can develop a modified cell, called an akinete. Akinetes contain large reserves of carbohydrates, and owing to their density and lack of gas vesicles, eventually settle to the lake bottom. They can tolerate adverse conditions such as the complete drying of a pond or the cold winter temperatures, and, as a consequence, akinetes serve as “seeds” for the growth of juvenile filaments when favorable conditions return. Heterocysts and akinetes are unique to the blue-greens.
Blue-greens in freshwater lakes
Unicellular and filamentous blue-greens are almost invariably present in freshwater lakes frequently forming dense planktonic populations or water blooms in eutrophic (nutrient rich) waters. In temperate lakes there is a characteristic seasonal succession of the bloom-forming species, due apparently to their differing responses to the physical- chemical conditions created by thermal stratification. Usually the filamentous forms (Anabaena species, Aphanizomenon flos-aquae and Gloeotrichia echinulata) develop first soon after the onset of stratification in late spring or early summer, while the unicellular-colonial forms (like Microcystis species) typically bloom in mid-summer or in autumn. The main factors which appear to determine the development of planktonic populations are light, temperature, pH, nutrient concentrations and the presence of organic solutes.
Attached and benthic populations in lakes
Many blue-greens grow attached on the surface of rocks and stones (epilithic forms), on submerged plants (epiphytic forms) or on the bottom sediments (epipelic forms, or the benthos) of lakes.
The epilithic community displays a clearly discernable zonation in lakes. Members of the genera Pleurocapsa, Gloeocapsa and Phormidium often dominate the dark blue-black community of the spray zone. Scytonema and Nostoc species form olive-green coatings and are more frequent about the water line, whilst the brownish Tolypothrix and Calothrix species are more typical components of the subsurface littoral community.
The epiphytic flora of lakes is usually dominated by diatoms and green algae, and blue-greens are of less importance in this community. Species of the genera Nostoc, Lyngbya, Chamaesiphon and Gloeotrichia have been occasionally encrusting submerged plants.
The epipelic community commonly includes blue-greens like Aphanothece and Nostoc particularly in the more eutrophic lakes. Benthic blue-greens growing over the littoral sediments and on submerged plants may be responsible for the occasional high rates of N2-fixation measured in oligotrophic lakes.
In the temperate region blue-greens are especially common in calcareous and alkaline soils. Certain species, Nostoc commune, are often conspicuous on the soil surface. Acid soils, however, lack blue-green element and are usually dominated by diatoms and green algae.
When viewed under the light microscope, blue-greens show a variety of movements, such as gliding, rotation, oscillation, jerking and flicking.
|The formation of water blooms results from the redistribution and often rapid accumulation of buoyant planktonic populations. When such populations are subjected to suboptimal conditions, they respond by increasing their buoyancy and move upward nearer to the water surface. Water turbulence usually prevents them reaching the surface. If, however, turbulence suddenly weakens on a calm summer day, the buoyant population may ‘over-float’ and may become lodged right at the water surface. There the cells are exposed to most unfavourable and dangerous conditions, like O2 supersaturation, rapidly diminishing CO2 concentrations and intense solar radiation, which are inhibitory to photosynthesis and N2-fixation, causing photo-oxidation of pigments and inflicting irreversible damage to the genetic constitution of cells. A frequent outcome of surface bloom formation is massive cell lysis and rapid disintegration of large planktonic populations. his is closely followed by an equally rapid increase in bacterial numbers, and in turn by a fast deoxygenation of surface waters which could be detrimental to animal populations within the lake. Water blooms are objectionable for recreational activities, and more importantly, create great nuisance in the management of water reservoirs.Most of these conditions are produced by just three blue-greens, informally referred to as Annie (Anabaena flos-aquae), Fannie (Aphanizomenon flos-aquae) and Mike (Microcystis aeroginosa). An oversupply of nutrients, especially phosphorus and possibly nitrogen, will often result in excessive growth of blue-greens because they possess certain adaptations that enable them to outcompete true algae. Perhaps the most important adaptation is their positive buoyancy, which is regulated by their gas vesicles which are absent in true algae.|
Their reputation as “nuisance” or “noxious” is totally undeserved. While periodic blooms are considered a nuisance in recreational lakes and water supply reservoirs of North America, the near continuous blooms of blue-greens in some tropical lakes are a valuable source of food for humans. Some blue-green species make major contributions to the world food supply by naturally fertilizing soils and rice paddies. R.N. Singh of the Banares Hindu University in India has shown that the introduction of blue-green algae to saline and alkaline soils in the state of Uttar Pradesh increases the soils’ content of nitrogen and organic matter and also their capacity for holding water. This treatment has enabled formerly barren soils to grow crops. Astushi Watanabe of the University of Tokyo found the introduction of Tolypothrix tenuis resulted in a 20% increase of rice crop. W.E. Booth of the University of Kansas showed through experiments in Kansas, Oklahoma and Texas, that a coating of blue-greens on prairie soil binds the particles of the soil to their mucilage coating, maintains a high water content and reduces erosion.
Humans also consume Spirulina. It contains all of the amino acids essential for humans, and its protein content is high (± 60%). It is a staple food in parts of Africa and Mexico. In China, Taiwan and Japan, several blue-greens are served as a side dish and a delicacy. Several areas in North America culture and commercially process certain blue-greens for various food and medicinal products such as vitamins, drug compounds, and growth factors.
Heterocystous blue-greens possess the unique ability to simultaneously evolve O2 in photosynthesis (in vegetative cells) and H2 by nitrogenase catalyzed electron transfer to H+-ions (in heterocysts), in the absence of N2 or other substrates of nitrogenase. This is the basis for the attempts of several workers to exploit the potential through the development of a `biophotolytic system’ for solar energy conversion, even though to date the thermodynamic efficiency has been disappointingly low.
Nevertheless, the utilization of blue-greens in food production and in solar energy conversion may hold immense potential for the future, and could be exploited for man’s economy. Progress in the study of the genetics of blue-greens may enable us to manipulate the N2-fixation (nif) and associated genes, and produce strains which fix N2, evolve H2 or release ammonia with great efficiency.
(Also see, Diverse taxa of cyanobacteria produce ß-N-methylamino-L-alanine (BMAA), a neurotoxic amino acid- Proc. the National Academy of Sciences of the USA, 2005)
|Poisonous blue-greens occur in ponds and lakes throughout the world. In Canada, they primarily occur in the prairie provinces. Poisoning has caused the death of cows, dogs, and other animals. Although humans ordinarily avoid drinking water that displays a blue-green bloom or scum, they may be affected by toxic strains when they swim or ski in recreational water bodies during a bloom. Typical symptoms include redness of the skin and itching around the eyes; sore, red throat; headache; diarrhea; vomiting; and nausea. The frequently occurring `swimmers itch’ is attributed to contact with Lyngbya majuscula, Schizothrix calcicola and Oscillatoria nigroviridis, which are commonly found in tropical and subtropical seawaters. The toxins responsible are lipid-soluble phenolic compounds. Since the same or similar symptoms can be produced by bacteria or viruses, one should not necessarily conclude that blue-greens are responsible for a human illness simply because the sick individual recently swam in a lake or pond that has suffered a bloom. Human death has not been documented. Reported cases affecting humans list Anabaena as the main culprit.Most of the recorded toxic blooms are caused by Microcystis aeruginosa, which manufactures “microcystin”, which yields 7 (or 14) amino acids upon hydrolysis. It causes enlargement and congestion of the liver followed by necrosis and haemorrhage, and may also exhibit neurotoxic activity. |
Alkaloid toxins (anatoxins, aphantoxins) act on the nervous system, leading to paralysis of muscles needed for breathing.
Two other genera, Oscillatoria and Nodularia are also known to produce toxic populations. Whether the animal survives the poisoning depends primarily upon the concentration of toxin ingested. Blue-green toxins may act on zooplankton and might be an effective mechanism of protection against grazing pressures.
Little is known about the percent of blooms that are toxic (upto 25% quoted in literature), and also why a toxic population is produced. A complicating factor is that part of a bloom can be toxic and another part nontoxic within the same lake. It has been suggested that toxic strains may develop only under a particular set of environmental conditions, or that toxin production may be associated with plasmid-mediated gene transfer.
Colour and identification
The blue-green color of cells (cyan means blue-green) is due to the combination of green chlorophyll pigment and a unique blue pigment (phycocyanin). However, not all blue-greens are blue-green. Their pigmentation includes yellow-green, green, grey-green, grey-black, and even red specimens. The Red Sea derives its name from occasional blooms of a species of Oscillatoria that produces large quantities of a unique pigment called phycoerythrin. In the arid regions of Central and East Africa, flamingos consume vast quantities of Spirulina, and their feathers derive their pink color from carotene pigments in filaments of Spirulina.
The blue-greens are microscopic life forms that exhibit several different types of organization. Some grow as single cells enclosed in a sheath of slime-like material, or mucilage. The cells of others aggregate into colonies that are either flattened, cubed, rounded, or elongated into filaments. Actual identification of cyanobacteria (blue-greens) requires microscopic examination of cells, colonies, or filaments, although experienced aquatic biologists can usually recognize Microcystis (colonies look like tiny grey-green clumps) and Aphanizomenon (green, fingernail-like or grass-like clippings).
Measures to control the growth of blue-greens
Chemicals are widely used to prevent the growth of nuisance algae, and the commonest one being copper sulphate. A number of other algicides are phenolic compounds, amide derivatives, quaternary ammonium compounds and quinone derivatives. Dichloron aphthoquinone is selectively toxic to blue-greens. The hazards of using toxic chemicals indiscriminately in the natural environment are well documented.
Biological control is in principle possible, though not always practical and as effective. Invertebrates like cladocerans, copepods, ostracods and snails are known to graze on green algae and diatoms. Daphnia pulex has been reported to feed on Aphanizomenon flos-aquae while present in the form of single filaments or small colonies but avoid large raft-like colonies. The copepod Diaptomus has been implicated in the grazing of Anabaena populations in Severson Lake, Minnesota.
Micro-organisms (fungi, bacteria and viruses) appear to play an important part in regulating growth of blue-greens in freshwaters. Certain chytrids (fungal pathogens) specifically infest akinetes, other heterocysts. Bacterial pathogens belonging to the group of Myxobacteriales can effect rapid lysis of a wide range of unicellular and filamentous blue-greens, though heterocysts and akinetes remain generally unaffected. Viral pathogens belonging to the group of cyanophages exhibit some degree of host specifity. Phage AR-1 attacks Anabaenopsis, phages SM-1 and AS-1 are effective against the unicellular forms, Synechococcus and Microcystis, Phage C-1 lyses Cylindropermum, and the LPP-1 virus is effective against strains of Lyngbya, Phormidium and Plectonema.
The long-term approach is no doubt the systematic removal of major nutrients.